Waht you need to know about marine sewage effluent testing

To reduce sewage’s contribution to marine pollution, the IMO spearheaded the very first global environmental project. Changes in both technology and policy have been steadily accruing over the past century, and this trend will only accelerate. Raw sewage being dumped into the ocean poses a health risk to marine life. Oxygen depletion and visual pollution caused by sewage are particularly problematic for countries that rely on tourism. Most human waste is disposed of in onshore facilities, like municipal sewers and treatment plants. Yet another source of maritime contamination is the dumping of raw sewage overboard by ships.

Sewage effluent can have disastrous consequences on ecosystems and human and marine health if not thoroughly screened.
Sewage discharge regulations, including those for ships’ equipment and systems for controlling sewage discharge, port reception facilities for sewage, and survey and certification requirements, may be found in Annex IV.
The oceans are thought to be able to process and deal with raw sewage on the high seas through natural bacterial action. Consequently, unless otherwise indicated, discharging sewage into the sea within a certain distance from the nearest land is prohibited by the provisions in Annex IV of MARPOL.
When sewage effluent is improperly released in coastal areas, it can deplete the oxygen in the water, killing off fish, coral, seaweed, and vital microorganisms that are important to the ecosystem’s survival.

Discharging raw sewage into the ocean is illegal unless the vessel is equipped with a functioning sewage treatment plant or is discharging comminuted and disinfected sewage using a permitted system more than three nautical miles from the nearest shore. It is permissible to release untreated sewage at sea, provided it is discharged more than 12 nautical miles from the nearest shore and the ship is moving at a speed of at least 4 knots, and the rate of untreated sewage release must be allowed by the administration per resolution MEPC.157(55).

Sewage testing on board is an easy way to spot problems before they become major headaches. If you wait until an issue arises, your system may be down for repairs, which is a waste of time and money.

It is acknowledged that the performance of sewage treatment plants may vary considerably when the system is tested ashore under simulated shipboard conditions or on board a ship under actual operating conditions.

Sampling should be carried out in a manner and at a frequency which is representative of the effluent quality. Figure below provides a suggested frequency for sampling, however, the frequency should take account of the residence time of the influent in the sewage treatment plant. A minimum of 40 effluent samples should be collected (over 10 days) to allow a statistical analysis of the testing data (e.g. geometric mean, maximum, minimum and variance).

Suggested hydraulic loading factors and sampling frequency for testing sewage treatment plants. May be modified as necessary to take account of characteristics of individual sewage treatment plants

MARPOL ANNEX IV – RESOLUTION MEPC.115 (51) entered into force on 27th September 2008. It applies to ships on international voyages which are either 400gt and greater or less than 400gt when certified to carry more than 15 persons, which includes passengers and crew.

As set out in Annex 22 Resolution MEPC.227(64) adopted on the 5th October 2012, sewage treatment plants installed prior to 1st January 2010, on ships other
than passenger ships operating in MARPOL Annex IV special areas and intending to discharge treated effluent into the sea, should comply with resolution MEPC.2(VI) adopted on 3rd December 1976.

As set out in Annex 22 Resolution MEPC. 227(64) adopted on the 5th October 2012, sewage treatment plants installed prior to 1st January 2016 and on or after
1st January 2010, on ships other than passenger ships operating in MARPOL Annex IV special areas and intending to discharge treated effluent into the sea, should comply with resolution MPEC.159(55) adopted on 13th October 2006.

As per MEPC.2(VI):

    • Coliforms – up to 250 CFU/100ml
    • Total Suspended Solids (TSS) – up to 100 mg/l
    • Biological Oxygen Demand (BOD) – up to 50 mg/l

As per MEPC.159(55):

    • Coliforms – up to 100 CFU/100ml
    • Total Suspended Solids (TSS) – up to 35 mg/l
    • Biological Oxygen Demand (BOD) – up to 25 mg/l
    • Chemical Oxygen Demand (COD) – up to 125 mg/l
    • Chlorine (Free) – up to 0.5 mg/l
    • pH – between 6.0 – 8.5

For exemplification purpose, we will describe the “Martek Marine” test kit. There are different testing kits,  produced by different companies for sewage, but the required testing is quite similar.

      The Permanganate Value test is used for indicating the general quality of final effluents as to its acceptability for discharge. You simply fill your sample containers and add Acidifying SE tablets. After a short 30 minute wait you read the result from the following table.

      It is possible to derive an indication of the Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) from the result of the Permanganate Value test.
      To convert the Permanganate Value (PV) for domestic sewage and effluent to probable BOD, COD and TOC values multiply by the following factors:

The probable BOD can be calculated from the result of the turbidity test using the following formula :

The Royal Commission Standard for Effluents recommends a BOD value of not more than 20 mg/l.

      The Turbidity test is designed to give a measure of the suspended solids content of the final effluent. It is also useful in following the day to day variation in the quality of sewage and effluent.
      The Turbidity test uses a specially calibrated plastic tube. This provides the simplest possible method of performing this important test. Test kit SP 304 includes a tube graduated at 30 to 500 turbidity units.

The Royal Commission Standards for Effluents recommend that the suspended solids content of sewage effluent should not be more than 30 mg/l.

    • PH TEST
      The pH test is carried out using a Universal pH test tablet in conjunction with a comparator colour wheel. The test covers the pH range 4 to 10.
      The expected pH range for raw sewage is 6 to 8.5,and the pH of final effluents
      should also fall within the 6 to 8.5 pH range unless other consent limits are specified.
      The Free Chlorine test is carried out by taking a sample of your sewage effluent, adding a crushed DPD 1 tabletvand comparing the sample effluent against the chlorine comparator disc. The corresponding number on the disc is what you record.
      A Bacteria Plate test simply involves taking a small 1ml sample of effluents and placing it onto the bacterial plate.
      You incubate the sample at 35oC for 48 hours and simply count the number of red colonies on the plates surface.
      You are looking for a result less than 100/250 CFU/ml depending on the vessel.
      The coliform test is carried out by taking a sample of your sewage effluent and adding Sodium Thiosulphate and ready cult. Once thoroughly shaken, you incubate the sample for 24 hours and 35oC. You then check for a colour change in the effluent sample, a change to blue/green indicates a presence of coliforms. Log result as present or absent.
      A check should be maintained on the temperature of effluent discharges and these should always be close to ambient temperatures. Sewage Effluent Test Kit SP 304 contains a 0 to 50°C thermometer complete in a brass protecting case.

In case that vessel requires to discharge untreated sewage, which means sewage that has not been treated by a type approved sewage treatment plant, or that has not been comminuted and disinfected, the rate of discharge should not exceed the rate described in the first table above at minimum permissible of 4 knots. Where the intended actual discharge rate exceeds that permissible at 4 knots, the actual discharge rate may need to be reduced or the speed increased.

Before undertaking a sewage discharge in accordance with this standard, the crew member responsible for sewage operations should ensure that the ship is en route, is more than 12 nautical miles from the nearest land and the navigation speed is consistent with the discharge rate that has been approved by the Administration. Ships with high discharge requirements are encouraged to keep notes of calculations of the actual discharges to demonstrate compliance with the approved rate.

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Source and Bibliography:

  • Martek Marine – Innovative ship solutions
  • MARPOL Annex IV

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